Surely, if you need to be signed off due to sickness, you should be signed off for whatever is actually causing that sickness. From what you've said, it doesn't sound as if you have a pregnancy-related illness, other than a bit of morning sickness, which is pretty normal. If that's the case, then you can't really get signed off for that. Obviously, if the vomiting is more severe (like HG), then that's a different scenario.
If it's actually your mental health that is making you unable to work, then you should get signed off for that reason. However, given the amount of time that you've had off to date, your employer may well start to question the sustainability of your employment. What were you hoping to get from the OH appointment? Are there specific reasonable adjustments that your employer could make that would facilitate your return to work.
Obviously, you'll be aware that pregnancy-related sickness cannot count against you in the same way as normal sickness absence, and perhaps that's why you're considering whether you could get signed off for this reason. However, please don't pursue this option unless you actually have a pregnancy-related illness, as it's taking the piss if you don't, and making things harder for other women in the process.